The European Telecommunications Standardisation Institute (ETSI) is currently in the process of standardising a new set of protocols for mobile telecommunications systems. The set of protocols is known collectively as the Universal Mobile Telecommunications System (UMTS). This third generation standard is also often referred to as 3GPP.
FIG. 1 illustrates schematically a UMTS network 1 which comprises a core network 2 and a UMTS Terrestrial Radio Access Network (UT 3. The UTRAN 3 comprises a number of Radio Network Controllers (RNCs) 4, each of which is coupled to a set of neighbouring Base Transceiver Stations (BTSs) 5. BTSs are sometimes referred to as Node Bs. Each Node B 5 is responsible for a given geographical cell and the controlling RNC 4 is responsible for routing user and signalling data between that Node B 5 and the core network 2. All of the RNCs are coupled to one another. A general outline of the UTRAN 3 is given in Technical Specification TS 25.401 V2.0.0 (1999-09) of the 3rd Generation Partnership Project, ETSI.
User and signalling data is carried between an RNC and a mobile terminal (referred to in UTRAN as User Equipment (UE)) using Radio Access Bearers (RABs). Typically, a UE is allocated one or more Radio Access Bearers (RABs) each of which is capable of carrying a flow of user or signalling data. RABs are mapped onto respective logical channels. At the Media Access Control (WAC) layer, a set of logical channels is mapped in turn onto a transport channel, of which there are two types: a “common” transport channel which is shared by different mobile terminals and a “dedicated” transport channel which is allocated to a single mobile terminal. One type of common channel is a Forward Access CHannel (FACH) which carries data in the downlink direction. Another type of common channel is the Random Access Channel (RAC) which carries data in the uplink direction. Several transport channels are in turn mapped at the physical layer onto a Secondary Common Control Physical CHannel (S-CCPCH) for transmission over the air interface between a Node B and a UE.
FIG. 2 illustrates certain of the layers present at a UE, a Node B, and an RNC of a UMTS network. In particular, FIG. 1 illustrates that the MAC layer, present at the RNC and the UE, is split into a MAC-c layer and a MAC-d layer.
Following the establishment of a user connection between a UE and the network, the network may decide to switch the connection from one channel type to another channel type. For example, a decision may be made to switch from a FACH/RACH channel to a DCH. The decision to switch is made by the MAC-c entity of the RNC. Upon allocation of a new channel, it is necessary for the WCDMA L1 layer of the UE to synchronise to the new (DCH) channel. Synchronisation makes use of one or more idle frames, or frames containing user data, sent to the UE. Once synchronisation has occurred, received frames are forwarded to the higher (L2) layers of the UE.
According to the current UMTS proposals, during this synchronisation phase, the MAC-d entity of the RNC may send user data to the Node B and the Node B informs the MAC-d entity whether the data was received by the Node B prior to or following L1 synchronisation of the UE. In the event that the data was received prior to synchronisation, the data is discarded by the Node B and the MAC-d entity of the RNC requests more user data from the RLC and sends this new data to the Node B. This loss of data will increase the bit error rate (BER).
As well as a possible loss of data, a transport channel switch will inevitably result in an interruption in the transmission of data between the core network and the UE, as the UE, is “ordered” to stop listening to the pre-existing channel just prior to synchronisation being initiated. In the case of a switch from a common channel to a dedicated channel, this interruption is particularly undesirable as it occurs precisely at that time when traffic intensity is likely to be high (such a switch is triggered by increased traffic intensity or by an expected increase). The transmission interruption occurring during the switch from a FACH/RACH to a DCH is illustrated in the signalling diagram of FIG. 3.